Real prices charge real money, and that money needs somewhere to go. Until your payouts setup with Stripe is complete, the price form shows a Complete onboarding link instead of letting you save.
How to fix it
Open Payouts. In Subscriptions, go to Payouts.
Complete onboarding. Click Complete onboarding and finish every step Stripe asks for.
Go back to your plan. Open the plan and save the price again.